FLAT....you are right and your mechanic is wrong. The axles should NOT turn when the wheels are spun, with the TRUCK RUNNING and NOT in 4x4. (You didnt state if the truck was running or not) If they are spinning, like you stated, and your truck was running, you need to diagnose where your problem is. It could be a bad IWE solenoid, bad check valve, leaking IWE, or leaking vacuum line. You need to get a vacuum guage to properly narrow down your problem. If you do not want to go that route, I would take it a dealership or another shop that understands the vacuum systems on these trucks, because if they dont, you might end up paying for alot more than what is actually wrong if the wrong place just starts throwing parts at it. If your mechanic really beleives that it is fine and that the spinning axles are normal, than i would definately not let him repair it, because he just proved to you that he doesnt understand these systems.

If you go the home diag. and repair, get a vacuum guage and take your time and read the beginning of the thread and the diagnostics. You will find your problem, it might sound tedious but the problem willl be obvious when you find it with the guage.

Let us know if you need any help.